1. Field of the Invention
The present invention relates to an active device array substrate, and more particularly to a thin film transistor (TFT) array substrate of a liquid crystal display (LCD) panel and a pixel structure thereof.
2. Description of Related Art
With the continuous advancement of larger-sized LCDs, the demand for progress and breakthroughs in wide-viewing-angle technology are growing to resolve the problem of viewing-angle resulted from larger display size. A multi-domain vertical alignment (MVA) LCD panel is one of the common LCDs utilizing wide viewing angle technology.
Generally speaking, the MVA LCD panel adopts a design of an alignment structure, such that the liquid crystal layer in the same pixel area is divided into various sections. Liquid crystal molecules in different sections are arranged in different directions, and thus such an LCD panel has a wider viewing angle. Due to the optical properties of the liquid crystal molecules, however, the conventional MVA LCD panel encounters difficulties of color distortion or insufficient color saturation when viewing an image from different viewing angles. This is the so-called “color washout”. To conquer said difficulties, a method for creating more domains in a pixel area has been proposed upon the improvement of the driving principles and the pixel design. The method is mainly performed by configuring another set of devices such as TFTs and pixel electrodes to compensate color distortion and insufficient color saturation. FIG. 1A depicts a design of the aforementioned pixel structure, and a brief introduction thereof is provided hereinafter.
As indicated in FIG. 1A, a pixel structure 120 is, for example, driven by a scan line 112 and a data line 114. A first TFT 130, a first liquid crystal capacitor Clc1, a first storage capacitor Cst1, a second TFT 140, a second liquid crystal capacitor Clc2, a second storage capacitor Cst2, and a coupling capacitor Cx are included in the pixel structure 120. It should be noted that the liquid crystal capacitors of the present invention refer to a capacitor structure constructed by a liquid crystal layer and electrodes disposed on the upper and lower sides of the liquid crystal layer. The first liquid crystal capacitor Clc1 and the first storage capacitor Cst1 are connected to the first TFT 130 so as to write a display voltage under the control of the first TFT 130. The second liquid crystal capacitor Clc2 and the second storage capacitor Cst2 are connected to the second TFT 140. The coupling capacitor Cx and the second TFT 140 jointly manipulate the written-in display voltage, leading to a different display voltage written in the first liquid crystal capacitor Clc1 and the first storage capacitor Cst1 from that into the second liquid crystal capacitor Clc2 and the second storage capacitor Cst2. Thereby, color distortion and insufficient color saturation are compensated.
However, as the LCD panel gets larger in size, a resistance-capacitance delay (RC delay) among the electronic devices of the LCD panel occurs more often. Particularly, since the second TFTs are additionally disposed in the single pixel area, the RC delay in said pixel structure is threatening. In more details, the second TFT far away from the driving terminal on the same scan line faces the problem of insufficient charging/discharging capacity on account of the RC delay. Thereby, the second liquid crystal capacitor Clc2 and the second storage capacitor Cst2 may function abnormally, and the display quality is then affected. The results of actual measurement are elaborated as follows.
FIG. 1B is a gamma curve depicting a conventional MVA LCD having the pixel structure 120. In FIG. 1B, curves 150, 160 and 170 respectively denote the gamma curves in the left section, the center section, and the right section of the LCD exhibiting images at a wide viewing angle. In the section where the LCD is close to the driving signal input terminal (i.e. in the left section), the charging time of the TFT is relatively sufficient. Accordingly, as shown in FIG. 1B, when the LCD in a medium gray scale state is displaying images at the wide viewing angle, the light transmittance rate in the left section (the curve 150) of the LCD is apparently higher than that in the center section and in the right section (the curves 160 and 170). Thereby, mura defects of the LCD panel occur when images are viewed at the wide viewing angle, leading to color distortion and insufficient color saturation. Accordingly, there is still room for further improvement of the display uniformity of MVA LCD